1. Field of the Invention
The invention relates to methods and systems for inducing gamma radiation emission and a gamma-ray laser system.
2. Description of the Prior Art
Stimulated emission of gamma radiation has never been unambiguously observed. Thus, it has always been assumed that the major goal of gamma-ray optics; namely, to produce a gamma ray laser, would require further fundamental research. If gamma-ray lasers could be developed there are many applications to; industrial processing, weapons development, communications, cancer treatment, as well as holographic observations of chemical processes, to mention a few. The reason for the usefulness of such a device is due to the coherence properties of laser light. However, gamma ray lasers do not, at present, exist. Of course ordinary laser action is based on stimulated emission. Without stimulated gamma emission, it is not possible to have gamma-ray lasing. As is well known, atomic physicists have already developed ordinary lasers, which arise from electromagnetic transitions in atoms. In fact, even x-ray lasers do exist now. For a gamma-ray laser, one must consider electromagnetic transitions in nuclei.
One of the important issues raised when considering the possibility of a gamma-ray laser is “lasing without inversion.” To summarize the point, if incident resonant radiation interacts with a system in which there are more resonant atoms in the ground state then in the excited state, there is more absorption than stimulated emission. Under this condition one cannot make a laser. The concept of “lasing without inversion” is based on the notion than one can, somehow, produce a ground state whose transition probability to go up to the excited state is zero while there still is a non-zero possibility of stimulated emission. Such ground states have been called “dark” states. Since the discovery of optical lasers, the scientific community has been interested and challenged to realize a gamma ray laser [see for example, Proc. of the Int. Gamma Ray Laser conference (GARALAS'97), Hyp. Int. 107 (1997); Hyperfine Interactions, Volume 107, Numbers 1-4, March 1997, pp 401-411: “Collective polynuclear superradiance rather than stimulated emission of Mössbauer radiation from 125m2Te and 123m2Te, G. A. Skorobogatov and B. E. Dzevitskii; Vysotskii et al, “Efficiency of excitation of highly active nuclear systems in the gamma-resonant medium heated by a laser pulse”, Plasma Physics Reports—December 1997—Volume 23, Issue 12, pp. 1046-1055; Collins et al, “Coherent and incoherent pumping of a gamma ray laser with intense optical radiation”, J. Appl. Phys., Vol/Issue: 53:7, Pages: 4645-4651, July, 1982; Baldwin et al, “Prospects for a gamma-ray laser”, Physics Today, vol. 28, February 1975, p. 32-39; Baldwin et al, Rev. Mod. Phys. 69, 1085-1118 (1997)].
A gamma ray laser would offer many applications because of the short wavelength and because of the high power density. Despite the considerable efforts of many groups, there still exists no idea of how to build such a device using present technology and our available knowledge of laser, nuclear and atomic physics.
The main problem is the realization of population inversion. Indeed, as recently as 2003, it has been postulated that gamma lasing without population inversion is impossible [see Coussement, et al, “Quantum optics with gamma radiation”, Europhysics News (2003) Vol. 34 No. 5]. For some time there has existed the so called gamma laser dilemma [G. C. Baldwin, J. C. Solem, and V. I. Goldanskii, Rev. Mod. Phys. 51, 4 (1981); G. C. Baldwin and J. C. Solem, Laser Phys. 5, 231, 326 (1995)].
To try to circumvent this dilemma, it had previously been thought necessary to follow one of two avenues. They have in common the storage of energy in long-lived isomers and finding a mechanism to release it on command. Storing large amounts of energy in nuclear isomers is not the real challenge, however. One can produce these long-lived isomers in nuclear reactors or with accelerator beams and separate them from other types of material by chemical and/or physical means. The technology is available in principle or at least there is enough knowledge available for it to be developed. The problem is the release of the stored energy ‘on command’ and in a very short time. It is on this point that the two approaches diverge conceptually.
One of the roads followed is to pump a long-lived nuclear isomer into an excited nuclear state via low energy X-ray irradiation. Subsequently, this excited state decays via the emission of gamma rays, representing a multiple of the input energy. In this scenario the long-lived isomeric state could act as a nuclear battery, in which energy is stored. A proof of principle for releasing the stored energy has been demonstrated using a K-isomer as the storage level [C. B. Collins et al., Phys. Rev. Lett. 82, 695 (1999); J. J. Carroll et al., Hyp. Int. 135, 3 (2001); C. B. Collins et al., Hyp. Int. 135, 51 (2001)].
From the point of view of nuclear models the result is surprising. Indeed, the K-isomeric state has a long lifetime because the K-selection rule hinders its decay into nuclear states at lower energy as this involves a large change in the projection of the angular momentum. It comes as a surprise that the transitions to higher energy states in this ground state band are then less affected by this hindrance. Although it is an interesting phenomenon for nuclear spectroscopists; because of the weak coupling of the X-ray pump with the possible lasing level, it is not yet an efficient mechanism for producing a gamma ray laser.
A prime condition for gain with stimulated emission is population inversion, or explicitly, the number of excited state nuclei from which lasing has to occur, exceeds the number of ground state nuclei. This condition requires that one be able to perform a nuclear isomeric separation in order to obtain nearly pure isomeric material. Such a technology could be developed in the future, for example, using the laser ionization method for producing isomeric enrichment, the separation being based on the difference in the hyperfine structure [U. Köster et al., Nucl. Instr. and Meth. in Phys. Res. B 160, 528 (2000)].
Even if a solid material could be prepared with most of the nuclei in the ground state as well as a large number in some long lived isomeric state, without having an inverted system, lasing might still be realized. To obtain lasing from such a system, the concept of “lasing without inversion” as introduced in quantum optics, could be translated to gamma radiation. In the optical range the effect of “lasing without inversion” has been demonstrated experimentally [C. Liu et al., Nature 409, 490 (2001); D. F. Phillips et al., Phys. Rev. Lett. 86, 783 (2001); A. B. Matsko et al., Advances in Atomic, Molecular and Optical Physics 46, 191 (2001)] and it was pointed out that the main new application will be the realization of lasers at very high frequency, for example UV, X-ray or even a gamma ray laser. The main point here is that one can create coherence in a three level system in such a way that absorption of the lasing frequency is cancelled by destructive interference while the emission, and in particular the stimulated emission, is not [R. Coussement et al., Phys. Rev. Lett. 71, 1824 (1993)].
In optics one can coherently excite two atomic levels by irradiating the atoms with two lasers of different frequency (color) but coupled in phase. Such phase-locked, two-frequency laser light is called ‘bichromatic light’ and can easily be obtained from one laser using frequency doubling or dividing crystals. However, an equivalent tool does not, at present, exist in gamma optics. Several unsuccessful attempts to produce stimulated gamma emission are described in U.S. Pat. Nos. 5,617,443; 4,939,742; 5,815,517; 5,887,008 and U.S. Application Publication no. 2002/0186805.
Ordinary laser action is based on stimulated emission. This occurs when incident light “stimulates” an atom to emit the same kind of light. An atom can emit light if a transition is made from a higher energy state to a lower energy state. The problem is, an atom can also absorb light with the same probability. If the light is absorbed, then none is available to provide stimulated emission. This is the reason for the term “population inversion”, that is used to explain ordinary lasers. If there are more atoms in the excited state than in the ground state, one can produce a laser. To summarize the point, if incident resonant radiation interacts with a system in which there are more resonant atoms in the ground state then in the excited state, there is more absorption than stimulated emission; hence one cannot make a laser under these conditions.
This population inversion appears to be very difficult to achieve using nuclei. However, the atomic physicists have also discovered another important result. This has been termed “lasing without inversion” [See for example, M. O. Scully and M. S. Zubairy, Quantum optics Cambridge University Press (1997) page 220]. Thus, suppose there is some way in which the incident radiation is not absorbed. Then, of course, this radiation can provide the required stimulated emission.
It is an object of the present invention to provide a gamma laser system and method and a method and system for producing and observing stimulated gamma radiation emission.